In the prior art it was desired to transmit and receive six bit comma free coded word commands back and forth between a vehicle and a station. These word commands had to be synchronized, since the six bits of each coded word were sent in serial form one word after another word. If the comma free code for a given word happened to be 101111, the transmitter would send 101111, 101111, and so forth repeatedly. When decoding the word on either the station or the vehicle, the decoding had to be synchronized as disclosed in U.S. Pat. Nos. 3,562,712; 3,992,698 and 4,015,082 with a recovered transmission clock in relation to a particular word. The previous practice to indicate the beginning of each bit in a word was to reverse the phase of the bit. For each bit whether it changed from a one to a zero, a zero to a one or if there were two ones or two zeros, every 18th of a second the phase was reversed, as shown by U.S. Pat. No. 3,551,889. A sinewave signal of a first frequency is transmitted for a ZERO and a sinewave signal of a second frequency is transmitted for a ONE. Each phase reversal indicates the beginning of another bit of the six bit comma free coded signal. It was the prior art practice in the transit industry in relation to command signal transfer to recover the source clock information, along with the binary data, and to synchronize the signal decoder with the source clock information. A circuit was provided to detect this phase reversal to recover the source clock information. Each bit lasted 1/18 second, with a typical frequency of 8880 hertz for the ZERO code and frequency of 5920 hertz for the ONE code. It is known in the prior art, as discussed in a published article entitled "Design Techniques For Automatic Train Control", by R. C. Hoyler in the July, 1972 issue of the Westinghouse Engineer, to send out information signals from the transmitter at a station or on a vehicle in the 5 to 10 kilohertz range and which signals are received by a receiver at the other of the vehicle or the station, including hardware circuits that demodulate the signal into the form of digital data. The BART train control apparatus performed such an operation. The input signal to the receiver is in the form of a 5 to 10 kilohertz FSK frequency shift keyed comma free code signal, but the demodulator takes that signal and converts it into digital data including ones and zeros, as disclosed in U.S. Pat. No. 3,992,698. The signal level is about five or six volts when it is a ONE and zero volts when it is a ZERO. The input signal line is going to be going high and low representing the ONE and ZERO bit signals that are received continuously, and it is desired to know what the meaningful code signal is out of that data stream.
For the example of vehicle and station door control, it is desired to recover out of this received data stream the coded word information that was originally embedded in it by the station transmission to the vehicle. A typical door control system transmits from the station to the vehicle four command word bit patterns, namely to open alighting doors, to open boarding doors, to open both doors, and to close both doors. For this purpose, four different code word commands are transmitted to the vehicle, with the first code word command being to close all doors. Since there are doors on both sides of the car, this coded word command signal closes doors on both sides of the car. Whenever a car pulls into a station lobby, one side of the car and station is designated as alighting for people setting off and one is designated as boarding for people getting on. The people come in the car from one side and go off the car from the other side. The alighting doors are opened first, which means the people on the car will start to exit toward one side of the car because those doors open first, and after everybody starts moving out then the boarding doors are opened a few seconds later. In that way people move off the car and move on the car through the vehicle and station doors at the same time to speed up the people transfer process. Typically, signals are first sent to open alighting doors, then signals are sent to open both doors, and then signals are sent to close both doors. The station initially sends these signals to the vehicle and the vehicle in turn sends these same code signals back to the station.
There is provided a decoding apparatus and method to interpret a binary code pattern of ones and zeroes that does not need an additional signal at the receiver representing the clock frequency of the source originating the received signal command.